Dynamic sub-sea cables are power cables that may be connected to, for example, floating platforms by means of which oil and/or gas is exploited from sub-sea wells. These cables may be freely extending in the sea water between certain fixing points, for example the platform and the sea bottom, and will therefore be moving with the motions of the sea, including sea currents and wind-induced motions. All materials but specifically the metals in such a cable are set out for mechanical fatigue due to these movements. Certain materials present a lower fatigue strength and will, therefore, possibly crack if subjected to a certain degree of fatigue strain. Especially the lead sheath, an often used barrier in subsea cables, is sensitive to fatigue and will eventually crack after too many or to large movements. Still, water ingress is not accepted by the cable and oil/gas industry. For that reason, lead might not be the best choice as water barrier material when it comes to dynamic power cables that are supposed to be freely moving in the sea.
Welded, soldered or glued copper sheaths perform better than corresponding lead sheaths with respect to mechanical fatigue. However, the density of copper is much less than that of lead, and as a result thereof cables using a copper sheath will be less dense than corresponding cables that use a sheath made of lead. In other words, the weight/diameter ratio of the copper sheath cable will be less than the weight/diameter ratio of a corresponding lead sheath cable. The applicant has realised that, for cables that are supposed to move freely and dynamically in open sea water, a too low cable density may be a drawback since such a cable might move too much and too easily when subjected to the motions of the sea. A well-designed cable should be able to follow the motions of the waves and the sea currents in such a way that its movement do not become so large that there might be a chance of getting a natural frequency behaviour of said cable.